Abstract

Geological characterizations, modeling and 3-D computer-generated visualizations of the Ordovician St. Peter Sandstone at the Hillsboro Gas Storage field in Montgomery County, Illinois, are discussed. Petrophysical analyses reveal four distinct hydraulic flow units in six cored wells. Furthermore, four lithologies, identified by thin section petrography, are associated with the various hydraulic units. Fieldwide visualizations of 3-D distributions of petrophysically-derived attributes - reservoir quality index (RQI) and flow zone indicator (FZI) - show considerable vertical variability but lateral continuity. This finding explains why it is easier to expand the gas bubble laterally than vertically. Advantages of the 3-D reservoir description of Hillsboro Gas Storage field include 1) improved definition of the spatial porosity distribution which leads to better estimation of reservoir volumetrics; 2) improved definition of reservoir hydraulic flow zones; and 3) development of realistic reservoir model(s) for the simulation and management of the gas storage field.

Introduction

The Hillsboro gas storage facility is located in sections 28, 29, 32 and 33 10N-3W and sections 4 and 5 9N-3W in Montgomery County, Illinois (fig 1) Gas is stored in the middle Ordovician St. Peter Sandstone at depths of approximately 3150 feet where a dome provides structural closure. Illinois Power Company began storing gas in the field in 1974 in a project that covers over 4000 surface acres. According to records at the Illinois State Geological Survey there are currently 12 injection/withdrawal wells, 9 observation wells and 1 salt water disposal well in operation. Eighteen (18) wells, consisting of 14 wells in the gas bubble and four water observation wells, are used in the geologic modeling.

The purpose of this study was to investigate the geology of the Hillsboro storage field, characterize the St. Peter Sandstone reservoir and incorporate this information into an integrated computerized geologic and engineering model. The model would then serve as a tool for understanding the gas storage performance and as an input into a reservoir simulation model.

Interpretation of the depositional environment of the St. Peter Sandstone is not part of this study but has been reported by other workers. Will man et al. (1975) reported that the St. Peter Sandstone in Illinois is a near shore transgressive deposit.

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